Publications (110)161.79 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: We study the pinning dynamics of magnetic flux (vortex) lines in a disordered typeII superconductor. Using numerical simulations of a directed elastic line model, we extract the pinning time distributions of vortex line segments. We compare different model implementations for the disorder in the surrounding medium: discrete, localized pinning potential wells that are either attractive and repulsive or purely attractive, and whose strengths are drawn from a Gaussian distribution; as well as continuous Gaussian random potential landscapes. We find that both schemes yield power law distributions in the pinned phase as predicted by extremeevent statistics, yet they differ significantly in their effective scaling exponents and their shorttime behavior.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We employ Monte Carlo simulations to investigate the nonequilibrium relaxation properties of the two and threedimensional Coulomb glass with different longrange repulsive interactions. Specifically, we explore the aging scaling laws in the twotime density autocorrelation function. We find that in the time window and parameter range accessible to us, the scaling exponents are not universal, depending on the filling fraction and temperature: As either the temperature decreases or the filling fraction deviates more from halffilling, the exponents reflect markedly slower relaxation kinetics. In comparison with a repulsive Coulomb potential, appropriate for impurity states in strongly disordered semiconductors, we observe that for logarithmic interactions, the soft pseudogap in the density of states is considerably broader, and the dependence of the scaling exponents on external parameters is much weaker. The latter situation is relevant for flux creep in the disorderdominated Bose glass phase of typeII superconductors subject to columnar pinning centers.Physical review. E, Statistical, nonlinear, and soft matter physics. 03/2014; 90(31).  [Show abstract] [Hide abstract]
ABSTRACT: In order to model real ecological systems one has to consider many species that interact in complex ways. However, most of the recent theoretical studies have been restricted to few species systems with rather trivial interactions. The few studies dealing with larger number of species and/or more complex interaction schemes are mostly restricted to numerical explorations. In this paper we determine, starting from the deterministic meanfield rate equations, for large classes of systems the space of coexistence fixed points at which biodiversity is maximal. For systems with a single coexistence fixed point we derive complex GinzburgLandau equations that allow to describe spacetime pattern realized in two space dimensions. For selected cases we compare the theoretical predictions with the pattern observed in numerical simulations.03/2014; 47(16).  [Show abstract] [Hide abstract]
ABSTRACT: We consider the nonconserved dynamics of the Ising model on the twodimensional square lattice, where each spin is influenced preferentially by its East and North neighbours. The singlespin flip rates are such that the stationary state is Gibbsian with respect to the usual ferromagnetic Ising Hamiltonian. We show the existence, in the paramagnetic phase, of a dynamical transition between two regimes of violation of the fluctuationdissipation theorem in the nonequilibrium stationary state: a regime of weak violation where the stationary fluctuationdissipation ratio is finite, when the asymmetry parameter is less than a threshold value, and a regime of strong violation where this ratio vanishes asymptotically above the threshold. The present study suggests that this novel kind of dynamical transition in nonequilibrium stationary states, already found for the directed Ising chain and the spherical model with asymmetric dynamics, might be quite general. In contrast with the later models, the equaltime correlation function for the twodimensional directed Ising model depends on the asymmetry.Journal of Statistical Mechanics Theory and Experiment 01/2014; 2014(5). · 1.87 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Systems with a bulk firstorder transition can display diverging correlation lengths close to a surface. This surface induced disordering yields a special type of surface criticality. Using extensive numerical simulations we study surface quantities in the twodimensional Potts model with a large number of states $q$ which undergoes a discontinuous bulk transition. The surface critical exponents are thereby found to depend on the value of $q$, which is in contrast to prior claims that these exponents should be universal and independent of $q$. It follows that surface induced disordering at firstorder transitions is characterized by exponents that depend on the details of the model.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: When four species compete stochastically in a cyclic way, the formation of two teams of mutually neutral partners is observed. In this paper we study through numerical simulations the extinction processes that can take place in this system both in the well mixed case as well as on different types of lattices. The different routes to extinction are revealed by the probability distribution of the domination time, i.e. the time needed for one team to fully occupy the system. If swapping is allowed between neutral partners, then the probability distribution is dominated by very longlived states where a few very large domains persist, each domain being occupied by a mix of individuals from species that form one of the teams. Many aspects of the possible extinction scenarios are lost when only considering averaged quantities as for example the mean domination time.Journal of Statistical Mechanics Theory and Experiment 07/2013; 2013(08). · 1.87 Impact Factor 
Article: Erratum: Surface Criticality at a Dynamic Phase Transition [Phys. Rev. Lett. 109, 175703 (2012)]
Physical Review Letters 06/2013; 110(23). · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In order to better understand the interplay of partnership and competition in population dynamics, we study a family of generalized MayLeonard models with N species. These models have a very rich structure, characterized by different types of spacetime patterns. Interesting partnership formations emerge following the maxim that “the enemy of my enemy is my friend”. In specific cases cyclic dominance within coarsening clusters yields a peculiar coarsening behavior with intriguing pattern formation. We classify the different types of dynamics through the analysis of the square of the adjacency matrix. The dependence of the population densities on emerging pattern and propagating wave fronts is elucidated through a Fourier analysis. Finally, after having identified collaborating teams, we study interface fluctuations where we initially populate different parts of the system with different teams.Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 03/2013; 87(3). 
Article: Dynamic phase transition in the threedimensional kinetic Ising model in an oscillating field
[Show abstract] [Hide abstract]
ABSTRACT: Using numerical simulations we investigate the properties of the dynamic phase transition that is encountered in the threedimensional Ising model subjected to a periodically oscillating magnetic field. The values of the critical exponents are determined through finitesize scaling. Our results show that the studied nonequilibrium phase transition belongs to the universality class of the equilibrium threedimensional Ising model.Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 03/2013; 87(3).  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the nonequilibrium relaxation properties and steady states of interacting magnetic flux lines in typeII superconductors in the presence of driving external currents and / or different types and configurations of pinning centers. We model the vortices as elastic lines, and study the competing effects of thermal fluctuations, mutual repulsion, and pinning to defects. We employ both threedimensional Monte Carlo and more efficient Langevin molecular dynamics simulations. Comparison of the resulting data for the nonequilibrium stationary states as well as the preceding relaxation regimes allows us to validate the utilization of both algorithms in outofequilibrium settings. We furthermore carefully analyze finitesize effects.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Understanding why and how species coexist is a necessary step to the program of manipulating multispecies environments in order to preserve the biodiversity of the environment of interest. To this end we consider a generalization of the cyclic competition of species model. We show that our model enjoys a Zn symmetry which is explained via a simple graph theoretic technique. This symmetry gives rise to pattern formation and cluster coarsening of the species. We show that biodiversity is achievable in the mean field limit provided that the species in the clusters have reaction rates which correspond to nontrivial equilibria.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Although the meanfield solution for four species in cyclic competition is generally in good agreement with stochastic results, it fails to describe the extinction and absorbing states that finite size systems inevitably fall into. We study the effects of dimension, lattice type, and swapping rate between particles on the time it takes for the system to go into a static absorbing state, which consists of a neutral species pair. Lattice types discussed are the well mixed environment, the onedimensional chain, the Sierpinski triangle, and the twodimensional square lattice. Data presented were acquired with simulations that have around the order of a thousand lattice sites or less, to capture finite size effects. The formation of domains composed of neutral species yields long lived states which promote coexistence.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: From complex biological systems to a simple simmering pot, thermodynamic systems held out of equilibrium are exceedingly common in nature. Despite this, a general theory to describe these types of phenomena remains elusive. In this talk, we explore a simple modification of the venerable Ising model in hopes of shedding some light on these issues. In both one and two dimensions, systems attached to two distinct heat reservoirs exhibit many of the hallmarks of phase transition. When such systems settle into a nonequilibrium steadystate they exhibit numerous interesting phenomena, including an unexpected ``freezing by heating.'' There are striking and surprising similarities between the behavior of these systems in one and two dimensions, but also intriguing differences. These phenomena will be explored and possible approaches to understanding the behavior will be suggested.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Using extensive Monte Carlo simulations we study the properties of the nonequilibrium phase transition encountered in driven threedimensional Potts systems with magnetic friction. Our system consists of two threedimensional blocks, coupled through boundary spins, that move along their boundaries with a constant relative velocity. Changing the number of states in the system from two (Ising case) to nine states, we find different scenarios for the surface behavior depending on whether the bulk transition is continuous or discontinuous. In order to fully assess the properties of this nonequilibrium phase transition, we vary systematically the strength of the coupling between the two blocks as well as the value of the relative velocity. For strong couplings between the blocks the phase transition is found to be strongly anisotropic.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: In this article we review the relation between string theory and nonequilibrium physics based on our previously published work. First we explain why a theory of quantum gravity and nonequilibrium statistical physics should be related in the first place. Then we present the necessary background from the recent research in nonequilibrium physics. The review discusses the relationship of string theory and aging phenomena, as well as the connection between AdS/CFT correspondence and the Jarzynski identity. We also discuss the emergent symmetries in fully developed turbulence and the corresponding nonequilibrium stationary states. Finally we outline a larger picture regarding the relationship between nonperturbative quantum gravity and nonequilibrium statistical physics. This relationship can be understood as a natural generalization of the wellknown Wilsonian relation between local quantum field theory and equilibrium statistical physics of critical phenomena. According to this picture the AdS/CFT duality is just an example of a more general connection between nonperturbative quantum gravity and nonequilibrium physics. In the appendix of this review we discuss a new kind of complementarity between thermodynamics and statistical physics which should be important in the context of black hole complementarity.International Journal of Modern Physics A 01/2013; 28(7). · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recently, different numerical studies of coarsening in disordered systems have shown the existence of a crossover from an initial, transient, powerlaw domain growth to a slower, presumably logarithmic, growth. However, due to the very slow dynamics and the longlasting transient regime, one is usually not able to fully enter the asymptotic regime when investigating the relaxation of these systems toward equilibrium. We here study two simple driven systemsthe onedimensional ABC model and a related domain model with simplified dynamicsthat are known to exhibit anomalous slow relaxation where the asymptotic logarithmic growth regime is readily accessible. Studying twotimes correlation and response functions, we focus on aging processes and dynamical scaling during logarithmic growth. Using the timedependent growth length as the scaling variable, a simple aging picture emerges that is expected to also prevail in the asymptotic regime of disordered ferromagnets and spin glasses.Physical Review E 01/2013; 87(11):012114. · 2.31 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We employ an elastic line model to investigate the steadystate properties and nonequilibrium relaxation kinetics of magnetic vortex lines in disordered typeII superconductors using Langevin molecular dynamics (LMD). We extract the dependence of the mean vortex line velocity and gyration radius as well as the meansquare displacement in the steady state on the driving current, and measure the vortex density and height autocorrelations in the aging regime. We study samples with either randomly distributed pointlike or columnar attractive pinning centers, which allows us to distinguish the complex relaxation features of interacting flux lines subject to extended vs. uncorrelated disorder. Additionally, we find that our new LMD findings match earlier Monte Carlo (MC) simulation data well, verifying that these two microscopically quite distinct simulation methods lead to macroscopically very similar results for nonequilibrium vortex matter.Physics of Condensed Matter 11/2012; 86(5). · 1.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Symmetries represent a fundamental constraint for physical systems and relevant new phenomena often emerge as a consequence of their breaking. An important example is provided by space and timetranslational invariance in statistical systems, which hold at a coarsegrained scale in equilibrium and are broken by spatial and temporal boundaries, the former being implemented by surfaces  unavoidable in real samples  the latter by some initial condition for the dynamics which causes a nonequilibrium evolution. While the separate effects of these two boundaries are well understood, we demonstrate here that additional, unexpected features arise upon approaching the effective edge formed by their intersection. For this purpose, we focus on the classical semiinfinite Ising model with spinflip dynamics evolving out of equilibrium at its critical point. Considering both subcritical and critical values of the coupling among surface spins, we present numerical evidence of a scaling regime with universal features which emerges upon approaching the spatiotemporal edge and we rationalise these findings within a fieldtheoretical approach.EPL (Europhysics Letters) 10/2012; 100(4). · 2.26 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In order to elucidate the role of surfaces at nonequilibrium phase transitions, we consider kinetic Ising models with surfaces subjected to a periodic oscillating magnetic field. Whereas, the corresponding bulk system undergoes a continuous nonequilibrium phase transition characterized by the exponents of the equilibrium Ising model, we find that the nonequilibrium surface exponents do not coincide with those of the equilibrium critical surface. In addition, in three space dimensions, the surface phase diagram of the nonequilibrium system differs markedly from that of the equilibrium system.Physical Review Letters 10/2012; 109(17):175703. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using extensive Monte Carlo simulations we study aging properties of two disordered systems quenched below their critical point, namely the twodimensional randombond Ising model and the threedimensional EdwardsAnderson Ising spin glass with a bimodal distribution of the coupling constants. We study the twotimes autocorrelation and spacetime correlation functions and show that in both systems a simple aging scenario prevails in terms of the scaling variable $L(t)/L(s)$, where $L$ is the timedependent correlation length, whereas $s$ is the waiting time and $t$ is the observation time. The investigation of the spacetime correlation function for the randombond Ising model allows us to address some issues related to superuniversality.Physics of Condensed Matter 07/2012; 85(9). · 1.28 Impact Factor
Publication Stats
950  Citations  
161.79  Total Impact Points  
Top Journals
Institutions

1970–2014

Virginia Polytechnic Institute and State University
 Department of Physics
Blacksburg, Virginia, United States


2012

University of Lorraine
 P2M  Physique de la Matière et des Matériaux
Nancy, Lorraine, France


2011

University of Luxembourg
 Theory of Soft Condensed Matter Physics
Letzeburg, Luxembourg, Luxembourg 
Clarkson University
 Department of Physics
Potsdam, NY, United States


2009

National Institute for Theoretical Physics
Stellenbosch, Western Cape, South Africa


2000–2007

FriedrichAlexander Universität ErlangenNürnberg
 Institute of Theoretical Physics
Erlangen, Bavaria, Germany


2006

Bielefeld University
 Faculty of Physics
Bielefeld, North RhineWestphalia, Germany 
University of Florence
Florens, Tuscany, Italy
